Couldn't be ELT be used to track its position? And don't airliners have ACARS to identify a crash?
Yes, the aircraft (9M-MRO) flying MH370 had 4 Emergency Locator Transmitters (ELTs) on board according to the final report:
9M-MRO had four ELTs installed. They were located as follows:
One FIXED ELT located above ceiling of the aft passenger cabin [...]
A control switch installed in the cockpit (flight deck) aft overhead panel provides the command signal. This switch is guarded in the ARMED position. If required, the flight crew can select the ELT to ON by moving the guarded switch from ARMED to ON. [...]
The ELT will activate upon a sudden deceleration force per the Technical Standard Order. [...]
One PORTABLE ELT located in the forward cabin right hand coat closet. This closet is used by the cabin crew. [...]
The portable ELT has a control switch on the front face. It is normally in the OFF position. When needed, the switch can be selected to the ON position to activate the ELT transmission. [...]
Two SLIDE RAFT mounted ELTs located at Door 1 Left and Door 4 Right (packed within the slide raft assembly). [...]
The slide raft ELT is automatically armed when the slide raft is deployed and inflated. Once armed the ELT is automatically activated by a water sensor coming in contact with water. [...]
No relevant ELT beacon signals from the aircraft were reported from the responsible Search and Rescue agencies or any other aircraft.
To answer your specific questions:
Couldn't be ELT be used to track its position?
No, the ELT is not normally transmitting anything, it is for emergencies only. As you can read in the quote above, only the fixed ELT will start transmitting automatically in case of a crash. There is a lengthy analysis in the final report about ELT effectiveness and the use of satellite tracking (have a look at chapter 1.6.7 for more details). As the report says, no signals were ever received from MH370.
And don't airliners have ACARS to identify a crash?
The ACARS is essentially just a network for sending text data via VHF or satellite. MH370 did send several ACARS messages during the first part of the flight. The system has nothing to do with identifying a crash. See chapter 1.9.4 for more details on ACARS for MH370.